Allow me to state my opinion regarding Cynthia. With the basic idea that we will lose either one or both N and Professor Sycamore in standard after rotation, this becomes a “prepare yourself” card. I do love how this card plays but to be fair, there is no method to answer this question if you intend to carry all three (N, Sycamore, Cynthia) until rotation in the deck. This being said, most of what I have seen and tried has been a minimum of 3.
Dealing with Glaceon GX… UGH! With the Volcanion Deck its more about figuring out how to get him out of active position then anything. Volcanion is so dependent on its abilities (as are so many other decks in the format) that Glaceon GX becomes Garbotoxin on steroids. I believe, in my opinion, the reason we are seeing so many viable abilities on non-GX cards in Ultra Prism is because of Glaceon GX. So other then get it out of active I truly have no idea, YET.